1. Field of the Invention
This invention relates to a method for forming a halftone screen for color scanners of a flat bed type and the like and to an apparatus therefor, and more particularly to a method and an apparatus for forming a halftone screen with an arbitrary number of screen lines by a system which exposes and scans a light spot of a predetermined size at a predetermined pitch, and outputs the result from a recording section to thereby form 4-color separated halftone images. The 4-color separated halftone image formation is conducted by superposing an image signal which is obtained by scanning an original comprising color images of continuous-tone with a halftone screen signal generated electrically so that the original is reproducible in the four colors of C (cyan), M (magenta), Y (yellow) and K (black).
2. Description of the Prior Art
In the fields of printing and plate making, an image scanning/reading/recording system is widely used in recent years as they can electrically process image information of an original and prepare an original film plate for printing to thereby rationalize the work process and improve the image quality.
Such image scanning/reading/recording system comprises basically an input section, a control section and an output section. More particularly, the input section of the system picks up image signals by a lighting system, a color separation system and a photometry mechanism, and converts them into the image information photoelectrically. The control section arithmetically processes the image information for gradation correction, color correction, contour emphasis, conversion from R (red), G (green), B (blue) to C, M, Y, K and the like in accordance with plate-making conditions. Then, the processed image information is converted again at the output section into optical signals such as laser beams to record images on a recording carrier comprising a photosensitive material. Next, the recording carrier is subsequently developed by a predetermined developing device and used as the original film plate for printing and so on.
When the original is of continuous tone images such as a picture or a photograph, it is necessary to separate the original in dots to express the degree of density. The continuous tone image is converted into a halftone image which is a group of dots formed in different dimensions in accordance with the degree of density of the image. There has been proposed a method for separating an original into dots by placing a contact screen comprising dots in different gradations in a net form on a film and exposing the recording carrier with optical signals corresponding to the original continuous tone images. In the image scanning/reading/recording system, a halftone screen which is a counterpart of such contact screen may be formed electrically.
As a preferable example of the halftone screen formation method of the prior art, Japanese Patent Publication (KOKOKU) No. 49361/1977 will be briefly described.
FIG. 1 shows a prior art wherein the reference numeral 100 denotes a basic periodic section of an electrically formed halftone screen. The halftone screen is formed by repetition of the same pattern and the minimum unit of the pattern is the basic periodic section 100. The basic periodic section 100 comprises eight scanning lines S.sub.1 through S.sub.8 which are arranged in the direction of Y in parallel to each other. Each part of the section 100 is defined with voltage signals unique to the scanning lines S.sub.1 through S.sub.8 which respectively change along the recording direction X. The respective voltages of the scanning lines S.sub.1, S.sub.2, S.sub.4 and S.sub.5 are set at high voltages when they are passing a point A to a point D in a dot 101 while the voltage of the scanning line S.sub.3 at the point E is set low. The voltages of the scanning lines S.sub.1 through S.sub.5 are set to gradually decrease from the points A through D toward the point E. The voltage signals of the scanning lines S.sub. 1 through S.sub.8 can form halftone screen signals of the respective scanning lines by superposing a plurality of alternating voltage signals of different periods and of a triangle shape, and by gradually shifting the phase thereof for each scanning line.
When multi-colored images are converted into dots for reproduction, it is necessary to generate a plurality of halftone screens and to superpose thus formed halftone gradation images. Each halftone screen is rotated by a predetermined angle .theta. from the recording direction X in order to prevent generation of Moire pattern when the halftone images are superposed.
The basic periodic section 100 is generated periodically at a frequency sufficient to cover all the scanning zones of the original to form the halftone screen. The halftone screen signals which form the halftone screen are superposed with the image signals which are read out from the original optically at the input section of the image scanning/reading/recording system to form the halftone images on the original film plate.
A halftone plate for color printing of an area modulation type can be identified with the number of screen lines (e.g. line number/inch, LPI), screen angle (m/n) and dot patterns. The screen angle is a rational number defined by m/n shown in FIG. 1, and is necessary for each of the four colors of C, M, Y and K. The screen angle and the dot pattern may be formed by the technique disclosed in the above Japanese Patent Publication No. 49361/1977. The screen line number may be obtained arbitrarily for the color scanner of a drum type by simply changing the magnification with an optical zoom. However, it is not easy to use an optical zoom for the color scanner of a flat bed (plane) type. Due to its high speed processing, the color scanner of a flat bed type is required to conduct main scanning of light spots optically, and an addition of a zooming mechanism to the system is almost impossible. The control system will further become complicated and larger, if the size of a light spot or pitch is changed.